1. Field of the Invention
The present invention relates to a system and method for controlling stopping and starting of a vehicle engine.
2. Background Art
The introduction of new vehicle types—e.g., hybrid electric vehicles—has enabled a number of systems and methods for saving fuel and reducing emissions. Hybrid electric vehicles can save fuel and reduce emissions through a number of methods, such as downsizing the engine, collecting regenerative power during braking, and implementing an engine stop/start function.
The engine stop/start function involves controlling the stopping and starting of the engine while the vehicle is in operation. In a conventional vehicle, the engine runs continuously once it is started by the driver. The engine provides torque to drive the vehicle and a number of onboard devices, such as compressors and pumps. The engine also provides torque to drive the alternator, which generates electricity to charge the battery and to run various electrical subsystems within the vehicle. Depending on different driving conditions, the engine may run at high speeds—-i.e., 3,000-7,000 revolutions per minute (rpm)—or at very low speeds, perhaps as low as a few hundred rpm.
Unless the driver specifically shuts the engine off, the engine in a conventional vehicle will continue to run even when the vehicle is completely stopped. While the vehicle is stopped, the engine continues to run, or idle, partly because certain vehicle subsystems continue to require power. For example, the transmission oil pump and power steering fluid pump require power even when the vehicle has stopped. In addition, certain driver-controlled subsystems, such as air conditioning and personal electronics, require power. The primary reason for keeping the engine running, even when the vehicle is stopped, is so that the vehicle is ready to move when the driver commands it to do so.
Even though the engine runs at low speeds when it idles, it nonetheless continues to burn fuel and exhaust emissions. Typically, the engine idle speed is kept as low as possible because the engine consumes less fuel at a lower speed. Thus, it would be desirable to shut the engine down if all of the vehicle and driver needs can still be met. Of course, when there are driver or vehicle requests, the engine would need to be able to start without any noticeable disturbance to the driver or the vehicle. This function may generally be called the stop/start function of the engine.
In a conventional vehicle the engine is started by a relatively small motor, called the starter, which is often powered by a battery. In such a vehicle, it may be hard to implement an engine stop/start function because the capacity of the battery may not be large enough to supply sustained power to all of the vehicle subsystems when the engine is shut down. Moreover, a typical starter may not be able to start the engine quickly enough to respond to a driver's demand to move the vehicle.
New developments in battery technology have increased the battery capacity while reducing the battery size and weight, thereby making it possible to meet the entire driver and vehicle system needs with electric power for a limited amount of time. In addition, the advances in motor technology and power electronics make it practical for the vehicle to use more powerful motors and use them more efficiently. Depending on the configurations, a battery and motor system in a vehicle with today's technology can start the engine almost instantaneously, or even drive the vehicle independently. All these advances make it viable to develop the engine stop/start function.
There are many possible configurations, and thus, many classifications for hybrid electric vehicles. Two broad classifications are mild hybrids and full hybrids. The distinction is whether the motor alone can drive the vehicle. In a mild hybrid electric vehicle, the motor is not powerful enough to drive the vehicle. The engine, once stopped, has to be started if the driver wants to launch the vehicle. By contrast, the motor in a full hybrid electric vehicle can drive the vehicle, though there may be limits on the vehicle speed that can be achieved. An engine stop/start function can be a major contributor for saving fuel and reducing emissions, whether the vehicle is a mild hybrid, a full hybrid, or a non-hybrid vehicle that has an engine and a second power source. Thus it is desirable to have a systematic approach for controlling the engine stopping and starting for various vehicle configurations.
One attempt to provide a drive control system for a hybrid vehicle is found in U.S. Pat. No. 6,201,312 issued to Shioiri et al. on Mar. 13, 2001. Shioiri et al. describes a control system for improving the fuel economy of a hybrid vehicle. Specifically, the control system provides for running the vehicle either with an electric motor or an internal combustion engine, where the selection between the two is made on the basis of a comparison of the two fuel economies. The control system of Shioiri et al. calculates an overall vehicle power demand, and uses this calculation to determine whether it is more efficient to run the vehicle with the motor or the engine. When it is determined that it is more fuel efficient to run the vehicle with the motor, the engine is stopped and the motor is used to run the vehicle. This is only done, however, if the battery state of charge is at least at a predetermined minimum level.
What Shioiri et al. does not consider is that there may be times when it is undesirable to stop the engine, even if it is more fuel efficient to run the vehicle with the motor. For example, it may be undesirable to have the engine cycle on and off too frequently. Thus, an improved control system would provide a mechanism for minimizing the possibility of a short engine on/off cycle. Moreover, if an electrical fault is detected in the motor, it may not be available to run the vehicle after the engine is stopped. Similarly, it may be desirable to consider driver inputs, such as a brake pedal position or gear shift position, when deciding whether to stop the engine. It may also be useful to examine other vehicle subsystems not only for their power consumption, but also for other operating conditions.
Another attempt to provide a controller for a hybrid electric vehicle is described in U.S. patent app. Pub. No. 20020019687, published by Suzuki, et al. on Feb. 14, 2002. Suzuki et al. describes a vehicle controller and vehicle control method for a hybrid electric vehicle. A number of conditions are examined to determine when to start and stop the vehicle engine. For example, if the “startability”of the engine is degraded, a flag may be set inhibiting the stopping of the engine in an idle state. The determination of whether the engine startability is degraded is based on systems external to the engine, rather than the engine itself. For example, the state of the battery charge may be examined, or whether an engine starting system—-e.g., an electric motor—has failed.
Accordingly, there exists a need for a system and method for controlling a vehicle such that it is possible to examine a number of different operating conditions, including conditions of the vehicle engine, and compare these conditions to corresponding predetermined conditions, and to stop the engine only if one or more of the operating conditions matches its corresponding predetermined condition.